Reader now resolves ActivityPub links internally instead of navigating
to external instances. Actor links open the profile view, post links
open a new post detail view with thread context (parent chain + replies).
External links in post content get rich preview cards (title, description,
image, favicon) fetched via unfurl.js at ingest time with fire-and-forget
async processing and concurrency limiting.
New files: post-detail controller, og-unfurl module, lookup-cache,
link preview template/CSS, client-side link interception JS.
Includes SSRF protection for OG fetching and GoToSocial URL support.
